7940G SIP to send keepalive/SIP Options

[CODNetadmin]

Hi Guys!

Is there any way for the SIP 7940G to send SIP Option message or keepalives?

Setup is 7940g <-->NAT <--> PBX

As checked,it only sends REGISTER message every 1 hour, and no SIP OPTION or keepalive which we suspect the reason why the connection drops.

Thanks!